1. Field of the Invention
The present invention relates to a fuel cell stack including electrolyte electrode assemblies and separators stacked alternately in a stacking direction. Each of the electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes. Fluid passages extend through the fuel cell stack in the stacking direction, and at least one of a coolant and reactant gases flows through the fluid passages.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane (electrolyte) interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between a pair of separators. The membrane electrode assembly and the separators make up a power generation cell for generating electricity. In use, a predetermined number of the power generation cells are stacked together. Further, terminal plates, insulating plates, and end plates are provided at opposite ends in the stacking direction to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In the fuel cell, a fuel gas flow field for supplying the fuel gas to the anode, and an oxygen-containing gas flow field for supplying the oxygen-containing gas to the cathode are provided in the surfaces of the separators. Further, a coolant flow field is provided between the separators for allowing the coolant to flow along the surfaces of the separators.
In general, in the so-called internal manifold type fuel cell, fluid supply passages and fluid discharge passages extend through the separators in the stacking direction. The fluids, i.e., the fuel gas, the oxygen-containing gas, and the coolant are supplied to the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field through the respective fluid supply passages, and discharged from the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field through the respective fluid discharge passages.
In the internal manifold type fuel cell, the terminal plates or the end plates also have the fluid supply passages and the fluid discharge passages as necessary. In this case, metal plates (metal components) such as the metal separators or the terminal plates contact the water produced in the reaction or the coolant water. Therefore, corrosion current flows through the metal plates easily, and electrical corrosion occurs in the metal plates undesirably.
In an attempt addressing the problem, for example, Japanese Laid-Open patent Publication No. 2002-124292 discloses a fuel cell stack as shown in FIG. 37. A terminal plate 3 is interposed between a separator 1 and an insulating plate 2. The fluid passage 4 extends through the separator 1, the terminal plate 3, and the insulating plate 2 in the stacking direction, and a stopper 5 is provided in the inner circumference of the terminal plate 3, around the entire circumference of the fluid passage 4.
An insulating grommet 6 is attached to the terminal plate 3. The insulating grommet 6 has an engagement portion 7 attached to the stopper 5. Further, the insulating grommet 6 has seal lips 8 at positions where the insulating grommet 6 contacts the adjacent separator 1 and the adjacent insulating plate 2.
In Japanese Laid-Open Patent Application No. 2002-124292, the grommet 6 having a complicated shape is used as an insulating structure, and the stopper 5 is formed in the inner circumference of the terminal plate 3. Thus, it is not possible to provide the insulating structure economically. In particular, if a large number of the insulating structures are provided in the fuel cell stack, the production cost of the fuel cell stack is considerably high.
Further, for example, Japanese Laid-Open Patent Publication No. 2001-155761 discloses a structure for cooling fuel cells as shown FIG. 38. In the cooling structure, four fuel cell stacks 1a through 1d are connected in series. The cooling structure includes a supply member 2a for supplying a coolant to a coolant flow field (not shown) in each of the fuel cell stacks 1a through 1d. 
The supply member 2a includes an inlet pipe 3a as an inlet of the coolant, and an outlet pipe 4a as an outlet of the coolant. Mesh members 5a, 6a, made of electrically conductive material are provided at the inlet pipe 3a and the outlet pipe 4a. The mesh members 5a, 6a are electrically connected through an electrically conductive line 7a. The electrically conductive line 7a is connected to a reference electrode 8a having a potential of 0V through an electrically conductive line 7b, and connected to the ground through an electrically conductive line 7c. In this manner, the corrosion of other apparatuses connected to the cooling structure, and leakage of the electricity to the outside are prevented.
In Japanese Laid-Open Patent Publication No. 2001-155761, each of the fuel cell stacks 1a through 1d includes a plurality of cells connected in series. Thus, in particular, the corrosion current flows easily on the high potential side through metal components such as the metal separators. Thus, electrical corrosion occurs in the metal components.
Further, U.S. Pat. No. 4,371,433 discloses grommets or the like as insulators. Specifically, as shown in FIG. 39, a separator 1e has a chamber 2b at its central position. A plurality of projections 2c are provided in the chamber 2b to form a plurality of flow passages 2d extending vertically. Inlet manifolds 3b are provided at the lower end of the flow passages 2d, and a channel 4b is formed at the upper end of the flow passages 2d. An outlet manifold 5b extending vertically is connected to one end of the channel 4b. Further, the separator 1e has an outlet manifold 5c. The outlet manifold 5c extends vertically as same as the outlet manifold 5b. 
An insulating liner 6b is provided at each of the inlet manifolds 3b, and an insulating grommet 7d is attached to each of the outlet manifolds 5b, 5c. Passages 6c connected to the flow passages 2d are formed in the insulating liner 6b, and the insulating grommet 7d has an opening 7e connected to openings 4c. 
In this case of U.S. Pat. No. 4,371,433, when the power generation cells are stacked substantially horizontally, the water produced in the reaction or the condensed water is likely to stay at the inner bottoms of the inlet manifolds 3b and the outlet manifold 5b (hereinafter, simply referred to as the “fluid passages”). In the water, ions are eluded from components such as the membrane electrode assemblies and the seals, and the electrical conductivity is increased. Thus, in the fuel cell stack formed by a plurality of fuel cells, since the voltage is applied between the metal separators during power generation, the short circuit occurs easily between exposed metal portions of the metal separators through the water which stays in the fluid passage.
Since the liquid resistance between the exposed metal portion of the metal separator and the fluid passage is the lowest in the region, the most of the corrosion current flows through the exposed metal portion. Thus, the corrosion of the metal separator progresses, and elusion of the metal ions causes reduction in the number of ions exchanged through the solid polymer electrolyte membrane. As a result, the power generation performance may be lowered undesirably, and the degradation of the solid polymer electrolyte membrane itself occurs.